Polycarbonate moldings have long been known. The disadvantage of polycarbonate, however, is that it is not itself inherently UV-resistant. The sensitivity curve of bisphenol A polycarbonate exhibits the highest sensitivity between 320 nm and 330 nm. Below 300 nm no solar radiation reaches the earth, and above 350 nm this polycarbonate is so resistant that no further yellowing occurs.
To protect polycarbonate from the damaging influence of UV rays in the atmosphere, UV stabilizers are generally used which absorb UV radiation and convert it to harmless thermal energy.
It is advantageous for a lasting protection if the damaging UV radiation is effectively filtered out even before it reaches the polycarbonate surface, as is possible through the use of UV protection layers, for example coextruded layers containing UV absorbers, films containing UV absorbers or varnish (or paint respectively as an equivalent expression) containing UV absorbers, on polycarbonate.
Typical classes of UV absorbers which are known to be suitable for use for this purpose, are 2-hydroxybenzophenones, 2-(2-hydroxyphenyl)benzotriazoles, 2-(2-hydroxyphenyl)-1,3,5-triazines, 2-cyanoacrylates and oxalanilides.
The art relating to multi-layer products is noted to include the following.
EP-A 0 110 221 discloses sheets comprising two layers of polycarbonate, wherein one layer contains at least 3 wt. % of a UV absorber. According to EP-A 0 110 221 these sheets can be produced by coextrusion.
EP-A 0 320 632 discloses moldings made from two layers of thermoplastics, preferably polycarbonate, wherein one layer contains special substituted benzotriazoles as UV absorbers. EP-A 0 320 632 also discloses the production of these moldings by coextrusion.
EP-A 0 247 480 discloses multi-layer sheets in which, in addition to one layer of thermoplastics, a layer of branched polycarbonate is present, wherein the polycarbonate layer contains special substituted benzotriazoles as UV absorbers. The production of these sheets by coextrusion is likewise disclosed.
EP-A 0 500 496 discloses polymer compositions which are stabilized with special triazines against UV light and their use as an outer layer in multi-layer systems. Polycarbonate, polyesters, polyamides, polyacetals, polyphenylene oxide and polyphenylene sulfide are cited as polymers.
It has been found, however, that for many applications, particularly for outdoor applications requiring long-term optical clarity, such as glazing applications, the known UV-stabilised polycarbonate moldings display a still unsatisfactory long-term resistance to yellowing.
For such applications a polycarbonate molding must not yellow by more than Δ YI of 3 (YI=Yellowness Index)under irradiation of 30 MJ/m2 at 340 nm (corresponding to 10 years' outdoor weathering in Florida).
Weathering in this test is carried out in an Atlas Ci 5000 Weatherometer with a radiation intensity of 0.75 W/m2/mn at 340 nm and a dry/rain cycle of 102:18 minutes. The blackboard temperature is 70° C., the sample chamber temperature 55° C. and the air humidity 40%.
The object of the present invention is to provide polycarbonate moldings which under irradiation of 30 MJ/m2 at 340 nm yellow by no more than a Δ YI of 3 under the cited conditions. It was found that in order to achieve this combination of UV absorber and matrix must have an adequate absorbance and low degradation.